load: prove that signalc has at least 5x higher throughput than signald

context

• signald often takes up to 30 minutes to deliver all messages to channels with several thousand subscribers
• in this MR, we build a load testing framework to replicate the load of a channel with 1,000 subscribers and use it to demonstrate that signalc can transmit 1,000 messages in under a minute, therby validating the decision to continue working toward shipping it